An electromagnetic radiation sensor having, for example, an MSM (Metal-Semiconductor-Metal) structure, MIM (Metal-Insulator-Metal) structure, or MIS (Metal-Insulator-Semiconductor) structure formed by stacking thin films is widely used as a photosensor of a reader of a CD-ROM, DVD-ROM, or the like, a heat sensor using infrared radiation, or a motion detector. When electromagnetic radiation is incident on the surface of this electromagnetic radiation sensor, a signal of this electromagnetic radiation is converted into an electrical signal.
A thin-film electromagnetic radiation sensor, such as a photodetector or pyroelectric sensor, capable of detecting up to an infrared region sometimes uses the MSM structure or MIM structure. Examples of metals used in these structures are Ni, Cu, Cr, Pt, or Au, each having conductivity σ higher than 105 S/m. However, a thin-film electromagnetic radiation sensor having the MSM structure or MIM structure using these metals has the drawback that a large amount of incident electromagnetic radiation is reflected by a metal film.
Several proposals, therefore, are made to increase the absorption efficiency of electromagnetic radiation incident on a thin-film electromagnetic radiation sensor having the MSM structure or MIM structure. Examples are a method of forming an antireflection film on a metal film which functions as a top electrode, a method of decreasing the thickness of a metal film which functions as a top electrode to a few nanometers, and a method of giving a metal film a two-layered structure.
Unfortunately, the formation of the antireflection film increases the fabrication cost.
Also, as described in U.S. Pat. No. 6,399,946, in the method in which the thickness of the metal film which functions as a top electrode is decreased to a few nanometers, if the thickness of the metal film which functions not only as an absorbent but also as a top electrode decreases to a few nanometers, the mechanical strength of the film significantly lowers, and other problem arises. In addition, it is extremely difficult to evenly form a thin film having a thickness of a few nanometers within the range of area of square centimeter order.
Furthermore, as described in U.S. Pat. No. 6,399,946, the fabrication method of giving the metal film a two-layered structure is complicated, so the cost rises, or the yield lowers.
Japanese Patent Application Laid-Open No. 2001-304955 discloses an infrared detecting element having, on a top electrode, an infrared absorbing portion which contains a metal sulfide. However, even this infrared detecting element cannot have sufficient sensitivity if a metal, such as Au, having high conductivity is used as the top electrode. Also, the fabrication is complicated because a layer which serves as the infrared absorbing portion must be formed on the top electrode.
In addition, the conductivity of a transparent conductive oxide, such as an Sr oxide, In oxide, or Pb oxide, is very high, i.e., about 6.25×105 S/m, although it also depends on film formation conditions. If a film like these is used as the top electrode, a leakage current increases, and the trapping effect in a dielectric film rises.
Patent reference 1                U.S. Pat. No. 6,399,946        
Patent reference 2                Japanese Patent Application Laid-Open No. 2001-304955        